1. Field of the Invention
This invention relates to insulating polymeric compositions comprising polyimide siloxanes, especially polyetherimide siloxanes, and to electrical wire or cable provided with a layer of insulating or jacketing material formed from said compositions.
2. Introduction of the Invention
Polymeric compositions comprising polyetherimide siloxanes are known for a number of applications. EP-A-0407061, for example, describes a wire having an inner coating of a halogen free plastics material and a halogen free, hard flexible outer coating of a copolymer of, or a mixture of, a siloxane and a polyetherimide. The outer coating advantageously has the low flammability known to be associated with polyetherimides, although it is preferred to add a further outer later of poly-ether-etherketone to reduce still further the flammability and also to improve cut through and abrasion resistance and resistance to attack by fluids or gaseous chemicals. EP-A-0407061 also discloses blending unspecified amounts of polyphenylene ether or nylon with the polyetherimide-siloxane.
In another reference, EP-0307670, improved flammability is achieved by blending flame resistant polyetherimide siloxane polyetherimide copolymer blends with fluorocarbon polymers. The compositions described are particularly useful for aircraft panels and interiors. Although the materials have particularly good flame retardancy properties they do have the disadvantage of incorporating halogens, which are not desired, and indeed are often barred by legislation, for certain applications, because of the toxic nature of halogens if escaping during a fire.
EP-A-0323142 describes a ternary polymeric blend for use as wire insulation comprising a blend of polyarylene ether ketone with polyetherimide and silicone polyimide copolymer. Each of these polymeric components has excellent flame retardancy properties and the triblend similarly has excellent time retardancy. However disadvantageously all the components are expensive and the triblend similarly expensive.
Flame retardancy of polymeric compositions can conveniently be assessed by analysing the L.O.I. (Limiting Oxygen Index) of the polymers. This test is specified in ASTM D2863-1987. It determines the lowest percentage of oxygen needed to support burning of the polymer under test. A higher value of L.O.I. therefore indicates a material with higher flame retardancy. Specifically polymer compositions with a L.O.I. of at least 21% will not burn in air, and are preferred for certain applications. Where L.O.I.'s are referred to in the present invention, they are determined according to ASTM D2863-1987.